<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    body, ul{
      margin: 0;
      padding: 0;
    }
    body{
      background-image: url(../images/grid.png);
    }
    svg{
      background-color: rgba(255, 0, 0, 0.1);
    }
  </style>

</head>
<body>

  <svg width="300" height="300" xmlns="http://www.w3.org/2000/svg" >
    
    <!-- 
      stroke , 而不是 fill
     -->
    <line x1="100" y1="100"  x2="200" y2="100" 
      stroke="red" 
      stroke-width="5"
      stroke-linecap="round"
    >
    </line>

  </svg>

</body>
</html>